Periodic perturbations were used to evaluate the system stability and robustness of naphthalene biodegradation in a continuous flow stirred tank reactor (CSTR) containing a soil slurry. The experimental design involved perturbing the test system using a sinusoidal input either of naphthalene or non-naphthalene organic carbon at different frequencies during steady state operation of the reactors. The response of the test system was determined by using time series off-gas analysis for naphthalene liquid phase concentration and degradation, total viable cell counts, and gene probe analysis of naphthalene degradative genotype, and by batch mineralization assays. Naphthalene biodegradation rates were very high throughout the experimental run (95 to greater than 99% removed) resulting in very low or undetectable levels of naphthalene in the off-gas and reactor effluent. Attempts to reduce the rate of naphthalene biotransformation by either reducing the reactor temperature from 20 degrees C to 10 degrees C or the dissolved oxygen level (greater than 1 mg/L) were unsuccessful. Significant naphthalene biodegradation was observed at 4 degrees C. While variable, the microbial community as measured by population densities was not significantly affected by temperature changes. In terms of naphthalene biotransformation, the system was able to adapt readily to all perturbations in the reactor.
Abbreviations: ADAM -a disintegrin and metalloproteinase. BSA -bovine serum albumin. CHO -Chinese hamster ovary cells. CTLD -C-type lectin-like domain. FBS -fetal bovine serum. MMPmatrix metalloproteinase. PBMC -peripheral blood mononuclear cells. PFA -paraformaldehyde. sCD93 -soluble CD93. ABSTRACTEfferocytosis -the phagocytic removal of apoptotic cells -is essential for the maintenance of homeostasis and prevention of the inflammatory and autoimmune diseases which can follow the lysis of uncleared apoptotic cells. CD93 is a transmembrane glycoprotein previously implicated in efferocytosis and angiogenesis, and upon mutation, results in the onset of efferocytosis-associated diseases such as atherosclerosis and rheumatoid arthritis. CD93 is produced as a cell surface protein which is shed as soluble CD93, but it is unknown how CD93 mediates efferocytosis or whether its efferocytic activity is mediated by the soluble or membrane-bound form. Herein, we demonstrate that the membrane bound form of CD93 has no phagocytic, efferocytic, or tethering activity, whereas soluble CD93 potently opsonizes apoptotic cells but not a broad range of Gram-Negative, Gram-Positive or fungal microorganisms. Using mass spectrometry, we identified the αxβ2 integrin as the receptor required for soluble CD93-mediated efferocytosis, and via deletion mutagenesis determined that soluble CD93 binds to apoptotic cells via its C-Type Lectin-Like domain, and to αxβ2 by its EGF-like repeats. This bridging of apoptotic cells to the αxβ2 integrin markedly enhanced efferocytosis by macrophages, and could be abrogated by knockdown of αxβ2 integrin. Combined, these data elucidate the mechanism by which CD93 regulates efferocytosis and identify a previously unreported opsonin-receptor system utilized by the immune system for the efferocytic clearance of apoptotic cells.Clearly, there is data supporting three distinct and mutually exclusive models of CD93 efferocytic function. In this study, we directly asses these three models of CD93 function, using in vitro models of binding, efferocytosis and opsonization. These experiments disfavor the efferocytic receptor and tethering receptor models of CD93 function, and instead demonstrate a strong opsonic effect of sCD93 against apoptotic cells but not against a panel of microorganisms. Using mass-spectrometry we then identified αxβ2 integrin as the efferocytic receptor which recognizes sCD93 opsonized targets, with apoptotic cells "bridged" by the CD93 CTLD domain to αxβ2 integrin on phagocytes via the CD93 EGFlike domain. This represents a previously undescribed opsonin-receptor system which aids in the efferocytosis of apoptotic cell by phagocytes such as macrophages. MATERIALS AND METHODS MaterialsDH5a Escherichia coli was a gift from Dr. John McCormick (University of Western Ontario), Rhodotorula minuta and Candida glabrate were gifts from Dr. Andre Lachance (University of Western Ontario). Escherichia coli K29 + and K29 -, and Burkholderia cenocepacia were gifts from Dr. Susan Koval (University of Wester...
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